This invention relates to a method for forming a metal pattern in accordance with a pattern transfer process using a stamp.
Substrates having metal patterns formed thereon are used in a wide variety of applications including printed wiring boards, interdigital electrode substrates for sensors, flexible switches, battery electrodes, solar batteries, antistatic protective films, electromagnetic shield housings, integrated circuits, motor housings, and flat display panels.
Metallization on such substrates is traditionally carried out by vapor phase processes such as CVD and wet processes such as electroplating. Recently the electroless plating process is often employed.
Electroless plating was accidentally discovered by Brenner et al. in 1944 during electrolytic plating reaction in an aqueous solution of sodium hypophosphite. An electroless nickel plating process was reported in 1946 and a patent was granted to Brenner et al. See A. Brenner, J. of Research of N.B.S., 37, 1 (1946) and U.S. Pat. No. 2,532,283 (1950). Unlike electrolytic plating using an anode capable of feeding a metal, the electroless plating requires to replenish a metal salt and reducing agent which vary with the progress of metal deposition. The replenishing method was improved by G. Gutzeit et al (see G. Gutzeit et al., U.S. Pat. No. 2,658,841 (1953)). Such improved replenishment is now widely used in the industrial plating process. See W. H. Safranek, The Properties of Electrodeposited Metals and Alloys, 2nd Ed. American Electroplaters and Surface Finishers Soc., 1986. Since a metal is deposited under the action of a reducing agent, the electroless plating can form a continuous metal film even on non-conductive materials such as ceramics and plastics (see W. A. Alpaugh and C. Forks, U.S. Pat. No. 4,152,467 (1979)). However, the adhesion between the plated metal and the substrate largely depends on the type of substrate. Even when substrates are previously surface treated with carbon functional silanes (CF silanes) or surface roughened with acids or alkalis, some substrates allow for undesirable metal peeling.
Unlike polysiloxanes commonly used as heat resistant polymers in the industry, reducing silicon polymers such as polysilanes are quite interesting in that they possess UV absorption properties, high heat resistance, flexibility, and good thin film forming ability due to the metallic property and unique electron delocalization of silicon as compared with carbon.
The inventors discovered that when such a reducing silicon polymer is treated with a solution containing a salt of a metal having a standard oxidation-reduction potential of at least 0.54 volt, metal colloid readily deposits from the metal salt (Synthetic Metals, 97, 273, 1998). The inventors further disclosed in JP-A 10-326957 that by combining a thin film of reducing silicon polymer having such properties with metal colloid-catalyzed electroless plating, a metal film providing a firm bond to the substrate can be formed. Additionally, the inventors already proposed a method for forming a metal pattern on a substrate involving forming a pattern of reducing silicon polymer by a simple step such as a stamping, ink jet printing or lithographic printing technique. This method, however, is difficult to form a fine pattern of the micron order.
Patterning is generally carried out by the photolithography using resist which has developed in the semiconductor and micro-machining fields. This patterning method involves applying a resist to a substrate, exposing the resist coating to light through a mask, and etching or developing the resist coating by suitable means, thereby forming a fine pattern on the substrate. In general, a stepper is used for more efficient mass fabrication. The resolution depends on the wavelength of exposure light and the thickness of a resist film. To increase the degree of fineness, attempts have been made to reduce the wavelength of exposure light and the thickness of a resist film. Since such a process using an expensive stepper proceeds through a number of complicated steps by means of a precisely controlled apparatus, it is not readily implemented in a general purpose research laboratory.
As a result of the effort of reducing the film thickness to the limit in order to improve the degree of fineness, a molecular monolayer resist was recently developed. Since Colvert et al. formed in 1991 a molecular monolayer of organic silane compound using deep ultraviolet radiation of 193 nm, a number of molecular monolayers have been utilized in forming micro-patterns. See C. S. Dulcey, J. H. Georger, V. Krauthamer, T. L. Fare, D. A. Stenger, and J. M. Colvert, Science, 252, 551 (1991). The molecular monolayer used for forming a micro-pattern is known as a self-assembled monolayer (SAM) and utilizes the phenomenon that when a particular substrate is immersed in an organic solvent containing a specific molecule, a molecular monolayer spontaneously forms. See A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self Assembly, Academic, Boston (1991). The molecular species is used in a particular combination with a substrate, for example, organic silane compounds such as Rxe2x80x94Si(ORxe2x80x2)3 for metal oxide substrates such as SiO2 and Al2O3; organic sulfur compounds such as alkane thiols (Rxe2x80x94SH) and dialkyd disulfides (Rxe2x80x94SSxe2x80x94R) for metal substrates such as gold, silver and copper; and alcohols (Rxe2x80x94OH) and amines (Rxe2x80x94NH2) for platinum substrates.
Utilizing the self-assembled monolayer, Whitesides et al. proposed in 1994 a micro-stamping process for forming a micro-pattern by transferring an ink pattern onto a substrate using a silicone rubber stamp having indentations and protrusions, followed by etching. It was also reported to form a metal pattern by immersing a rubber stamp in a liquid dispersion of palladium colloid, pressing the rubber stamp against a substrate for transferring the palladium colloid on stamp protrusions to the substrate, and immersing the substrate in an electroless plating solution, whereby the metal deposits on only those areas where palladium colloid is attached. See A. Kumar, H. A. Biebuyck, G. M. Whitesides, Langmuir, 10, 1498 (1994), and Y. Xia, G. M. Whitesides, Angew. Chem. Int. Ed. Engl., 37, 575 (1998).
Nevertheless, this method could not use the stamp ink in a stable salt form such as palladium chloride, but the stamp ink in the form of palladium colloid which is formed by causing a reducing agent to act on palladium acetate. The palladium colloid is very unstable. Even when a surfactant such as tetraammonium halide is added for stabilizing the colloid, a uniform metal pattern is not always produced by stamping, owing to the influence of agglomerates and precipitates. Also the adhesion between metal and substrate largely varies with the type of substrate, so that accumulative strains are induced within a metal thin film depositing with the progress of plating, leading to the likelihood of peeling. See P. C. Hidber, W. Helbig, E. Kim, G. M. Whitesides, Langmuir, 12, 1375 (1996).
An object of the invention is to provide a method for forming a finely defined metal pattern having good adhesion though inexpensive simple steps without a need for exposure and development steps.
We have found that a finely defined metal pattern can be formed on any type of substrate at a firm bond to the substrate, by forming a thin film of a reducing silicon polymer on a substrate, immersing the substrate in a solution of a salt of a metal having a standard oxidation-reduction potential of at least 0.54 volt, typically palladium, silver or gold, forming a pattern of alkane thiol on the substrate by the micro-stamping process, and effecting electroless plating.